Scientific names: Nuttallia obscurata and Potamocorbula/Corbula amurensis

Featured image credit: User: pintail, Link to observation.

What Are They?

Nuttallia obscurata, commonly known as the purple varnish, or mahogany clam was first introduced to the United States and Washington from ship ballast water from Asia It is freshwater tolerant and is most abundant in areas where there is freshwater runoff.

Potamocorbula/Corbula amurensis, commonly known as Asian/Overbite Clam is native to the area spanning eastern Russia to southern China. It was first detected in San Francisco Bay, California in 1986.

Are They Here Yet?

The purple varnish clam has been reported in northern Puget Sound for about 10 years. Dense colonies can be found on the San Juan Islands in densities exceeding 800 per square meter.

The overbite clam has not been detected yet in WA, but it is close. It currently is known from the San Francisco Bay area.

Why Should I Care?

The purple varnish clam grows prolifically, forming dense colonies very rapidly and displacing native species. It can be harmful to humans because of its higher level of toxins.

The overbite clam also grows very quickly and can live in a variety of habitats. Its presence has been cited in the declines of recreational fishing stocks as well as native species.

How Can We Stop Them?

To prevent spread, clean, drain, and dry your equipment after each use.

What Are Their Characteristics?

  • Purple varnish clams
    • Purple varnish clams grow to less than 3 inches in length.
    •  The purple varnish clam gets its name from the shiny brown coating on the outer surface of the shell.
    • They have two, long, thin tubelike structures, called siphons, and a large foot that are translucent white and can be fully retracted. The foot helps the siphon burrow and live deep underground.
    •  The interior surface of the shell is usually a uniform shade of purple. Occasionally small snails will have purple coloring showing on the outer surface.
    • The varnish clam is nearly oval shaped, with the posterior end slightly longer and more angular. The shell is relatively flat, with a large external hinge ligament.
  • Asian/overbite clams
    • This is a tan, white, or yellow clam, up to about 25 mm long.
    •  The ‘overbite’ clam, derives its name from the right half of the shell being larger than the left and slightly overbiting its lower margin.
    • The shell has the overall shape of a broad isosceles triangle.
    • In younger clams the shell surface is covered by a thin, smooth tissue, but in older clams the tissue becomes wrinkled around the lower margin.
    • The overbite clam is primarily subtidal, but is occasionally abundant on intertidal mudflats.

Additional Photographs